In a typical refinery, crude oils are subjected to atmospheric distillation to produce lighter fractions such as gas oils, kerosenes, gasolines, straight run naphtha, etc. Petroleum fractions in the gasoline boiling range, such as naphthas, and those fractions which can readily be thermally or catalytically converted to gasoline boiling range products, such as gas oils, are the most valuable product streams in the refinery. The residue from the atmospheric distillation step is then distilled at a pressure below atmospheric pressure. This later distillation step produces a vacuum gas oil distillate and a vacuum reduced residual oil which typically contains relatively high levels of asphaltene molecules. These asphaltene molecules are responsible for most of the Conradson carbon residue and metal components in the resid. They also contain relatively high levels of heteroatoms, such as sulfur and nitrogen. These feeds have little commercial value, primarily because they cannot be used as a fuel oil owing to ever stricter environmental regulations. They also have little value as feedstocks for refinery processes, such as fluid catalytic cracking, because they produce excessive amounts of gas and coke. Also, their high metals content leads to catalyst deactivation. Thus, there is a great need in petroleum refining for greater utilization of such feedstocks for example by upgrading them to make them more valuable cleaner and lighter feeds.
A significant amount of feedstock in the gas oil boiling range is used to make olefins in steam cracking process units which contains a furnace comprised of fired tubes, or coils in which the feedstock is thermally cracked at temperatures of about 540.degree. C. to 760.degree. C. in the presence of steam. While gas oils are adequate feedstocks for such purposes, they are also relatively expensive feedstocks because of their preferred use for the production of transportation fuels. Residual feeds, which are substantially cheaper than gas oils, are typically unsuitable for use in steam crackers because of excessive cracking and coke formation in the furnace tubes leading to overheating and equipment plugging.
An attempt to overcome these problems was made in U.S. Pat. No. 2,768,127 which teaches the use of residual feedstocks for the production of aromatic and olefinic product streams. This was accomplished by contacting the residua feedstock in a fluidized bed of coke particles maintained at a temperature from about 675.degree. C. to 760.degree. C. While such attempts have been made to overcome these problems, there remains a need for improved processes having better control of solids and vapor residence times.